Selective catalytic reduction (SCR) is commonly used to remove NOx (i.e., oxides of nitrogen) from the exhaust gas produced by internal engines, such as diesel or other lean burn (gasoline) engines. In such systems, NOx is continuously removed from the exhaust gas by injection of a reductant into the exhaust gas prior to entering an SCR catalyst capable of achieving a high conversion of NOx.
Ammonia is often used as the reductant in SCR systems. The ammonia is introduced into the exhaust gas by controlled injection either of gaseous ammonia, aqueous ammonia or indirectly as urea dissolved in water. The SCR catalyst positioned in the exhaust gas stream causes a reaction between NOx present in the exhaust gas and a NOx reducing agent (e.g., ammonia) to reduce/convert the NOx into nitrogen and water.
In many applications, such as SCR systems for vehicles, for example, the storage of ammonia in the form of a pressurized liquid in a vessel may be too hazardous and a storage method involving absorption in a solid may circumvent the safety hazard of anhydrous liquid ammonia. For example, metal ammine salts are ammonia absorbing materials, which can be used as solid storage media for ammonia, which in turn, for example, may be used as the reductant in SCR to reduce NOx emissions from internal combustion engines in vehicles, see e.g., U.S. Pat. No. 8,088,201 and WO 1999/01205. The ammonia can be released from the ammine salts through thermal desorption, e.g., by external heating of a storage container, see e.g., id. and U.S. Patent App. Pub. NO. 2010/0086467. The ammonia is released from an either adsorptive or absorptive solid storage medium, among others Sr(NH3)8Cl2 or Ca(NH3)Cl2 in granular form, in a storage container and temporarily stored as a gas in a buffer volume. The amount of ammonia to be supplied to a reaction volume in the vehicle's exhaust system is dosed under the control of an electronic controller according to the current operating state of the engine.
In vehicular applications, the SCR system typically includes one or more main storage units and a smaller start-up storage unit. Heating devices are arranged to heat the main storage units and start-up storage unit separately to generate gaseous ammonia by thermal desorption. The amount of ammonia to be desorbed from the storage medium can, for example, be controlled by a feed-back control in which the pressure in the storage container is measured by a pressure sensor. The heater can be cycled on and off to maintain the pressure in the storage container at or near a target pressure. The start-up storage unit is generally much smaller than the main storage units so that it reaches the pressure threshold more rapidly than the main storage units and, accordingly, can begin supplying gaseous ammonia to the SCR system in a shorter period of time. However, due to its limited storage capacity, the start-up storage unit is not suitable as a long-term source of ammonia for the SCR system.
Simultaneously operating the heating units for the start-up storage unit and one or more main storage units may place excessive loads on the vehicle's electrical system. Accordingly, it is desirable to provide a method for modulating operation of the main and start-up storage units to reduce the load placed on the vehicle's electrical system.